An image forming apparatus causes a writing device to form on a photoreceptor (electrostatic latent image bearing member) an electrostatic latent image based on image information, and visualizes the electrostatic latent image with a toner (developer) so as to form a toner image (visible image). Then, a transfer device transfers the toner image from the photoreceptor to a sheet that is a recording material.
In the case in which the transfer device is a transfer roller, the toner image is transferred to the sheet by (i) supplying the sheet to a transfer nip portion where the photoreceptor and the transfer roller are compressed against each other, and (ii) conveying the sheet (recording material) by rotational forces of the photoreceptor and the transfer roller. Because a transfer voltage is applied to the transfer roller, the sheet passing through the transfer nip portion is electrically charged by the transfer voltage. Therefore, the toner on the photoreceptor is absorbed by the sheet.
Incidentally, a peripheral velocity of the transfer roller is higher than that of the photoreceptor. Therefore, the sheet once sticks to the photoreceptor, but is pulled due to the difference in peripheral velocity between the photoreceptor and the transfer roller. Thus, the sheet is separated from the photoreceptor. This arrangement is made to avoid deterioration in printing quality, such as hollow characters and half-tone thin dots caused due to a separation discharge generated when the sheet is separated from the transfer nip portion.
That is, the transfer voltage is applied to the transfer roller to transfer the toner to the sheet, however it is no exaggeration to say that a portion where the transfer voltage works normally is the transfer nip portion. Therefore, a white portion (that is, a portion on which the toner is not deposited) on the surface (close to the photoreceptor) of the sheet at the transfer nip portion is electrically charged with a high potential. On this account, when the sheet is separated from the transfer nip portion, the separation discharge is generated between the white portion of the sheet and a high potential portion on the photoreceptor. Due to the separation discharge, some of the toner transferred to the sheet is reversely transferred to the photoreceptor. This causes the above-described deterioration in printing quality.
In front of the transfer nip portion, a sheet conveying roller, called an idle roller, is provided. The sheet conveying roller rotates at substantially the same peripheral velocity as the transfer roller. As shown in FIGS. 17(a) to 17(d), a sheet P conveyed by an idle roller 116 is conveyed to a contact point of a transfer nip portion 127 in such a direction that the front edge of the sheet P proceeds toward an outer circumference of the photoreceptor 121. After the front edge of the sheet P first contacts with the photoreceptor 121, the sheet P is conveyed to the transfer nip portion 127 by the rotation of the photoreceptor 121.
If the front edge of the sheet P directly contacts with the contact point of the transfer nip portion 127, the sheet P vibrates at the moment of the front edge of the sheet P entering to the transfer nip portion 127. This vibration may cause a print slur (image deviation, transfer deviation) and/or a paper cockle at the front edge of the sheet P.
Further, in front of the transfer nip portion 127, a bended portion 128 of the sheet P is formed as shown in FIG. 17(d). The bended portion 128 is formed by substantially equalizing the peripheral velocity of the idle roller 116 and the peripheral velocity of a transfer roller 125. By forming the bended portion 128 in front of the transfer nip portion 127, the sheet P is conveyed to the transfer nip portion 127 in a state in which the sheet P surely sticks to the surface of the photoreceptor 121. Therefore, it is possible to prevent the problem in which, before the sheet P reaches the transfer nip portion 127, the sheet P sticks to the surface of the transfer roller 125 so as to be charged unnecessarily. Excessive charge to the sheet causes the above-described phenomenon of reversely transferring the toner.
By the bended portion 128 which intends to be flat, the sheet P is pushed in a direction in which the sheet P is conveyed. Therefore, the amount of the bended portion 128 is adjusted so that slipping of the sheet P is avoided by a nip pressure of the transfer nip portion 127.
Moreover, in recent years, a particle diameter of the toner for visualizing the electrostatic latent image has been reduced due to an increase in resolution of the image information. Conventionally, the particle diameter of the toner is substantially in a range from 8 Φμm to 12 Φμm. However, in recent years, the particle diameter of the toner is substantially in a range from 4 Φμm to 7 Φμm. In the case of a small-particle toner used in recent years, even if large particles and fine particles are removed in a manufacturing step, crushing occurs due to friction at the time of frictional electrification that is the application of electric charge to the toner. Therefore, the toner whose particle diameter is 2 Φμm or less also contributes to an image development.
Conventionally, the image forming apparatus forcibly omits a signal corresponding to a sheet peripheral edge portion determined by the image forming apparatus, from an image signal supplied from a terminal device such as a host computer, so as to form a blank space.
If the above omission is not carried out in the case of recording on the entire sheet the image based on the image signal supplied from the terminal device, the toner corresponding to the sheet peripheral edge portion of the toner image on the photoreceptor is not transferred, and the toner remains on the photoreceptor. Then, the remaining toner scatters inside the image forming apparatus. This causes deterioration in image quality and/or a jam.
With regard to such a technique for forcibly forming the blank space, for example, Japanese Unexamined Patent Publication No. 101769/1991 (Tokukaihei 3-101769, published on Apr. 26, 1991) discloses a technique for separately changing the size of each blank space corresponding to each edge of a sheet when images are formed on the same sheet twice. Even if an error in a tolerance range occurs, an image can be prevented from sticking out, and it is possible to increase a region which can be utilized effectively for image formation.
Moreover, Japanese Unexamined Patent Publication No. 068874/1997 (Tokukaihei 9-068874, published on Mar. 11, 1997) discloses a technique in which, after a first test pattern (a solid image having a small blank space at a rear edge) is outputted and an image whose rear edge portion is blurred is obtained, a second test pattern having a normal blank space at a rear edge is outputted and the blank space at the rear edge is adjusted so as to correct the blur at the rear edge portion of the image. With this, it is possible to prevent the damage caused by the transfer charge (transfer electric field) to the image carrier (photoreceptor), and also possible to obtain the image of high quality.
However, since the particle diameter of the toner has been reduced these days, there occur problems which had not occurred in the past. That is, the problem is a phenomenon in which the rear edge of the image formed on the sheet moves backward, that is, the image is lengthened on the sheet. In a terrible case, the blank space provided at the sheet rear edge portion completely disappears. This phenomenon relates to a printing ratio on the sheet, and occurs in the case in which the printing ratio is high.
As a result of studies for finding out the cause of the above-described phenomenon, the present inventors found that the phenomenon is caused by a phenomenon in which the sheet slips with respect to the photoreceptor at the transfer nip portion. The present inventors further found that this slipping is caused by a combination of the following factors: (i) a decrease in particle diameter of the toner, (ii) the difference in peripheral velocity between the photoreceptor and the transfer roller and (iii) the bended portion formed in front of the transfer nip portion.
That is, in the case in which the amount of toner between the sheet and the photoreceptor is large, the absorptive power between the sheet and the photoreceptor decreases due to the decrease in particle diameter of the toner. Because of the decrease in the absorptive power, the nip pressure of the transfer nip portion cannot overcome the pushing power generated by the bended portion formed in front of the transfer nip portion. Therefore, the sheet moves in accordance with the peripheral velocity of the transfer roller. As a result, the sheet slips with respect to the photoreceptor.
The following will explain a mechanism of the decrease in the absorptive power between the sheet and the photoreceptor in reference to FIGS. 16(a) and 16(b). FIGS. 16(a) and 16(b) show the transfer nip portion where the toner image is transferred. A conventional large-particle toner T is used in FIG. 16(a), and a small-particle toner t of today is used in FIG. 16(b).
At the transfer nip portion 127, the photoreceptor 121 and the transfer roller 125 are compressed against each other via the toner (T, t) and a sheet P in this order when viewed from the photoreceptor 121, and a transfer voltage is applied by a transfer voltage applying section 129 through the transfer roller 125. The sheet P is conveyed in a sheet conveyance direction (indicated by an arrow X) by the rotational forces of the photoreceptor 121 and the transfer roller 125. Note that in FIGS. 16(a) and 16(b), an arrow Y indicates a rotation direction of the photoreceptor 121 and an arrow Z indicates a rotation direction of the transfer roller 125.
By applying the transfer electric field from the transfer roller 125 through the sheet P to the toner on the photoreceptor 121, the toner is absorbed by the sheet P. However, even in the case in which the thickness of a toner layer in FIG. 16(a) is the same as that in FIG. 16(b), an air layer in the toner layer made by the small-particle toner t is larger than an air layer in the toner layer made by the large-particle toner T.
Therefore, in the photoreceptor, the toner, the sheet and the transfer roller, the distance of propagation of the electric field is longer in the toner layer of the small-particle toner t than in the toner layer of the large-particle toner T. In the case in which the distance of propagation is long, the intensity of the electric field (electric field intensity) becomes low when the electric field propagates the toner layer and reaches the photoreceptor 121. As a result, the absorptive power between the sheet P and the photoreceptor 121 decreases.
Since the absorptive power between the sheet P and the photoreceptor decreases, the phenomenon of slipping of the sheet with respect to the photoreceptor occurs by the pushing power of the bended portion formed in front of the transfer nip portion. As a result, the phenomenon of backward movement of the rear edge of the image transferred to the sheet P occurs.
In the case in which the rear edge of the image moves backward and the blank space provided at the rear edge portion of the sheet completely disappears, there are problems in that the remaining toner on the photoreceptor causes printing stain when printing an image on the following sheet(s) and the printing quality (image quality) deteriorates because of no blank space. In addition to these, in a compact image forming apparatus which employs a switchback conveyance method and is capable of carrying out two-side printing, the sheet winds around a fixing roller and the jam occurs.
In the switchback conveyance method, a front edge and a rear edge reverse between when printing on a first surface and when printing on a second surface. That is, the rear edge portion of the first surface becomes the front edge portion of the second surface. In the case in which the blank space at the front edge portion disappears, the sheet is conveyed to a fixing process that is the next process of the transfer process and the unfixed toner is molten and fixed, the molten toner sticks to the fixing roller, the sheet winds around the fixing roller and the jam occurs.
This problem occurs since the particle diameter of the toner has been reduced. Therefore, this problem is a new problem which had not been considered in the past. Since the techniques disclosed in the above-described Japanese Unexamined Patent Publication Nos. 101769/1991 and No. 068874/1997 do not consider the problem, those techniques, of course, cannot solve the problem.